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Cubing planet
So, first of all, you should learn how to solve the cube. You can click here to find one of the easiest-to-learn tutorials. But now, if you have already learned how to solve the cube, everything depends on how fast you do it. Usually, after some practice, you will hit the 1-minute mark (or a minute and a half). After that, you can check the tips of how to become sub 50 by clicking here (sub means solving the cube in under 50 seconds for example). Later in the section "How to get faster at 3x3" you will find tips & tricks for getting faster and faster.
This is actually really important. As a first tip: avoid the Rubik's Brand Cube because it is not suitable for speedcubing. There are lots of cubes for speedcubing. There are two types of them: magnetic and non-magnetic. The magnetic ones are more stable, controllable and they feel way better than the non-magnetic ones. I recommend you getting a magnetic cube from the beginning, so you won't need to get another cube to improve your times. Also, there are stickered and stickerless cubes (they don't have stickers). I recommend the stickerless ones because the stickers of the stickered cubes are peeling off, which makes them pretty ugly.
These are the best magnetic cubes for beginners (prices are in USD):
$5.99
$7.99
$7.99
$8.99
$9.99
The fact that the QiYi Thunderclap V3 M is the most expensive in the list doesn't mean it is the best. To my opinion the best cube you can buy is the MoYu RS3 M 2020 and you can even use it to get really fast times even when you improve more.
This is the list of non magnetic cubes that I recommend:
$3.99
$3.99
$4.99
$3.99
$7.99
I am sorry that a few images are the same, it's because I couldn't find good pictures and the cubes are almost the same on the outside look.